In systems of the above type the tentering chains carry tenter bodies which in turn support the above mentioned tentering hooks or clamps. Additionally, the tenter bodies carry guide rollers which run along guide surfaces of the guide rails of the system. The tenter bodies are pivoted to the chain links and it is desirable that the chain pitch is adjustable. The term "chain pitch" in this context means the on-center spacing between two chain bolts that journal neighboring chain links and tenter bodies to each other.
Two systems are known for the stretching of synthetic material films. One system involves stretching the film separately in the longitudinal direction and then stretching the film separately in the crosswise direction, whereby the two stretching operations take place sequentially in a two-stage operation. In such a system it is not possible to change the pitch of the chain subsequent to the crosswise stretching.
The second of the above two mentioned systems involves the simultaneous stretching of the film in the longitudinal direction and in the crosswise direction. During the stretching operation the longitudinal stretching involves stretching steps corresponding to a multiple of the chain pitch.
In both conventional systems it is desirable to apply a controlled shrinking to the stretched foil to stabilize the synthetic material of the foil. The controlled shrinking is accomplished in the crosswise direction by guiding the tentering chains along tracks that converge subsequent to the crosswise stretching. The shrinking in the longitudinal direction can be accomplished in a system equipped with tentering chains as disclosed in U.S. Pat. No. 4,890,365 (Langer), corresponding to German Patent (DE) 3,716,603, disclosing a stepless or continuously adjustable chain pitch by means of bellcranks which interconnect neighboring chain links, whereby means are provided for adjusting the tilting of the bellcranks. However, such tilting of the bellcranks is limited so that the chain pitch can only be changed up to about 1.5% of the normal chain pitch.